The invention describes a method for covering the surface of a structure with a discharge plasma and, more particularly, methods of use of such plasma (1) to cover the surface of a body to affect the aerodynamic properties of the body by decreasing or increasing the drag of the body, and (2) to provide a plasma medium inside a plasma reactor used for treatment of work pieces, including semiconductor wafers used for fabrication of microelectronic devices.
The term xe2x80x9cplasmaxe2x80x9d usually describes a partially ionized gas composed of ions, electrons and neutral species. This state of matter can be produced by the action of either very high temperatures, or strong direct current (DC) or an alternating current, such as by a radio frequency (RF) electric field. Stars, nuclear explosions and electric arcs represent high temperature or xe2x80x9chotxe2x80x9d plasmas. Free electrons that are energized by an imposed DC or RF electric field and then collide with neutral molecules produce glow discharge plasmas. These neutral-molecule collisions transfer energy to the molecules and form a variety of active species which can include photons, excited atoms, metastables, individual atoms, free radicals, molecular fragments, monomers, electrons and ions.
Low power plasmas, such as dark discharges and coronas, have been used in the surface treatment of various materials. Because of their relatively low energy content, corona discharge plasmas can alter the properties of a material surface without damaging the surface.
Glow discharge plasmas represent a type of relatively low power density plasma. These glow discharge plasmas can produce useful amounts of visible and ultraviolet (UV) radiation. Glow discharge plasmas have the additional advantage therefore of producing visible and UV radiation in the simultaneous presence of active species.
In recent years, investigators have suggested that creating weak ionization, i.e., low-density non-equilibrium plasma, in from of aerodynamic vehicles can reduce drag. It is known that preheating a layer of air in front of vehicles can reduce drag, but such heating is generally considered energetically unfavorable. Blackburn et al. (U.S. Pat. No. 5,797,563, issued on Aug. 25, 1998) describe a method for increasing the aerodynamic and hydrodynamic efficiency of a vehicle by radiating tuned microwave electromagnetic energy outwardly form a vehicle.
The effect due to weak ionization can require much less energy than preheating the air. Drag reduction aerodynamic vehicles, particularly for supersonic vehicles at high altitudes, can be economically important even if drag is reduced by only a few percent. Roth (U.S. Pat. No. 5,669,583, issued on Sep. 23, 1997) describes a method and apparatus for using uniform glow discharge plasma to cover vehicles at approximately atmospheric pressure to control drag characteristics. Roth""s method utilizes applied frequencies from below RF to the very lower regions of RF, where RF frequencies are generally defined to be in the range of 10 kHz to 300,000 MHz.
For many years, plasmas have been used for etching and deposition of surface layers, such as in the fabrication of microelectronic and optoelectronic devices. The surface modification is caused by the fluxes of electrons, ions, neutrals, and radicals that leave the plasma and impact and react with the surface of the work piece. Because the chemistry and physics of surface modification is complex, it is advantageous to have separate control of the plasma density and the ion energies in order to optimize the processes.
Early plasma reactors formed plasma by applying RF potentials between the work piece and an opposing electrode. These early designs did not provide separate control of the plasma density and ion energies. Subsequently, inductively coupled plasma sources were developed for use in the microelectronics industry (e.g., U.S. Pat. No. 5,650,032, issued on Jun. 27, 1997). These plasma reactors generally use an induction coil powered by a first RF power supply to control the plasma density and a second, capacitively coupled RF power supply, to energize the work piece and control the ion energies.